One of the UK's finest and least known gardens will on Tuesday be unveiled in newly restored glory after decades hidden away from the general public's gaze.

The 90 acres of Wrest Park in Bedfordshire are unarguably magnificent, probably Britain's largest "secret garden", with surprises around every wooded corner. Although it is a nationally important garden its existence has remained virtually unknown.

John Watkins, head of gardens and landscape at English Heritage, said Wrest Park was unusual because it retained designs from the 17th century to the 20th century.

"You can literally walk through 300 years of garden history," he said. "It's this palimpsest of garden history that is so special, but also it is stunningly beautiful – so you can come here whether you want to delve into the history of the place or just look at it."

Wrest Park was owned by the De Grey family for nearly 700 years and there are three key stages in the landscape's history – the formal woodland garden created by Henry, Duke of Kent, in 1706; changes made under the direction of Jemima, Marchioness Grey, in the 18th century's latter half; and then the work of her grandson Thomas, Earl de Grey, from 1833.

Wrest Park opens to the public on Thursday, revealing the first fruits of an ambitious 20-year restoration plan. As visitors step out from the French-inspired mansion, designed by Thomas in the 1830s to replace the old, dark and dingy house he demolished, they are met by a long, Versailles-like view of the central gardens.

Within the grounds there are examples of work by some of the most famous names in English gardening and architecture history, including Thomas Archer, who designed a magnificent baroque pavilion in 1709-11.

Then there is the hand of the most famous of them all, Lancelot "Capability" Brown, who was hired by Jemima in 1758 to make the boundary canals less formal and more natural.

A great gardening pioneer he may have been, but it is clear that while at Wrest, Brown was the "hired help". The rusticated column that was erected for Brown is inscribed with the words: "These gardens originally layed out by Henry Duke of Kent were altered by Henry Duke of Hardwicke and Jemima Marchioness Grey with the professional assistance of Lancelot Brown, 1758, 1759 and 1760."

Closer examination of the column reveals cracks from where a big beech tree fell down in the 1990s, crushing the monument. "This was before we took over the site, and it highlighted to us the importance of getting Wrest Park into protection," said Watkins.

Other surprises in the gardens include a tucked-away bath house, built in 1770 to resemble a semi-ruined classical building. Inside, family and friends would have walked across the pebble and deer vertebrae floor, to step, probably quite slowly, into the cold water plunge pool.

Then there is a small dog graveyard with headstones for family pets down the years, and a good source of inspiration for anyone stuck for a name: Douba, perhaps? Or Freuah, Una, Little Dick, Dingey, Busy, Fury, Dorroch, Phedra, , Nissy, Kelpie, Tottie, Petsy or Pet.

Jemima was in charge of the gardens for a long time and comes across as an enlightened but slightly dotty matriarch. There is, for example, the Mithraic altar she devised with her husband, which has seemingly Persian and Greek text, but is no more than an intellectual joke.

After Thomas, the gardens were looked after well with the house remaining remained in the family until Auberon Herbert in 1905, who leased Wrest Park to the US ambassador. During the first world war it was used as a military hospital.

When Herbert, a liberal politician and captain in the Royal Flying Corps, died in action, the estate was sold to northern industrialist JG Murray who felled quite a lot of trees when things got financially tricky.

He sold it to Sun Alliance Insurance in 1939 and after the second world war it became a centre for modern agricultural engineering research.

English Heritage took over in 2006 and devised a restoration plan stretching over 20 years. It was helped by a £1.14m grant by the Heritage Lottery Fund. The first phase being seen this week includes the restored Italian and rose gardens, a new exhibition on its history and access to the miles of pathways and vistas set over the 90 acres.

Simon Thurley, English Heritage's chief executive, said: "Wrest Park tells the story of England's love affair with landscape. It is a unique place capturing 300 years of gardening history. So now with the successful completion of this first phase of restoration, Wrest Park can rightfully reclaim its place as one of the great gardens of England."